The present invention relates to methods of making cement.
A process of making cement is known in which a mixture of ground raw materials (raw meal) is preheated in a preheating cyclone, the preheated raw meal is calcined, the calcined raw meal is burned in a rotary kiln, the cement clinker formed in the rotary kiln is cooled and the cement clinker is ground.
Cement is an inorganic nonmetallic powder, which is mixed with water and, after that mixing, spontaneously hardens and remains permanently solid after it has hardened. A distinction is made between various so-called standard cements, which include Portland cement, iron Portland cement, blast furnace slag cement and trass cement. The standard cements produced in the Federal Republic of Germany have the following chemical composition: CaO 41 to 67% by weight, SiO.sub.2 18 to 33% by weight, Al.sub.2 O.sub.3 3 to 14% by weight, Fe.sub.2 O.sub.3 0.5 to 4.5% by weight, MnO 0 to 0.6% by weight, MgO 0.5 to 9% by weight and SO.sub.3 1 to 4.5% by weight.
The production of hydraulic cement includes the procuring and dressing of raw materials, the burning of the mixed raw materials to produce cement clinker, the production of the inter-grindable substances and the joint grinding of the clinker and optionally one or more inter-grindable substances as well as calcium sulfate as a setting and hardening control agent. The raw materials (limestone and clay) are ground in a dry state to produce the raw meal, which, during the grinding operation, is dried by a hot gas and is subsequently heated and thereafter burned to produce cement clinker Depending on the type of the kiln the raw meal is heated for about 1 to 5 hours to burning temperature of about 1450.degree. C. and is held at that temperature for about 10 to 20 minutes. Thereafter the cement clinker is cooled as rapidly as possible. As the raw meal, which may still have residual moisture content below 1%, is heated the adhering water is removed at temperatures up to about 100.degree. C. and the water which is adsorptively and chemically bound in the clay is removed at temperatures up to about 600.degree. C. In the presence of SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 the decomposition of the calcium carbonate begins at temperatures as low as 550.degree. to 660.degree. C. and proceeds very rapidly above 900.degree. C. During the burning the mixture loses about 35% by weight of its dry weight. In the Federal Republic of Germany, about 97% of the cement clinker is burned in rotary kilns, which have an inclination of 3 to 4 degrees. Because of the inclination and rotation of the kiln the preheated raw meal fed at the upper end flows toward a flame produced by a pulverized coal or oil or gas burner, which is provided at the lower end of the kiln. The mixture is heated to a temperature for 1350.degree. to 1500.degree. C., which is required for formation of clinker. In the vicinity of the flame the temperature is from 1800.degree. to 2000.degree. C. The burnt clinker leaves the rotary kiln and falls into a cooler, in which air is used as a cooling fluid and in which the clinker is cooled to a temperature of 800.degree. to 900.degree. C. The air that has been used to cool the cement clinker is supplied to the rotary kiln as combustion air. The raw meal is preheated either in the rotary kiln or in a separate preheater, which preferably consists of a unit comprising a plurality of cyclones. The hot exhaust gases from the rotary kiln flow through the cyclone preheater from bottom to top and the dry raw meal is added to the exhaust gases before the upper most cyclone stage. The dry raw meal is again removed from the gas in each cyclone and before the next cyclone stage is resuspended in the gas stream. In the preheater the raw meal is usually heated to a temperature of about 800.degree. C. The exhaust gas leaving the uppermost cyclone stage is still at temperatures of 300.degree. to 400.degree. C. The raw meal may already be calcined in part in the cyclone preheater. In the prior art the raw meal which has been preheated and slightly calcined in the preheater is calcined in a separately fired calciner, which desirably constitutes a cyclone and in which a major part of the calcining is effected. The calcined raw meal is supplied to the rotary kiln for burning of clinkers. The rotary kiln preforms in addition the remaining calcining. The heat requirement of the calciner is from 30 to 70% of the total heat required for burning the clinker. To produce cement, the cement clinker is ground alone or with inter-grindable substances like blast furnace sand, trass, oil shale or fly ash. Gypsum for controlling the setting and hardening is also added to the clinker which is to be ground
Because the cement clinker is produced at very high temperatures, the rotary kiln is heated with comparatively high-grade fuels, which also afford the advantage that, because of their relatively low ash content, they only slightly change the composition of the raw meal. The use of low-grade, high-ash fuels is difficult, because their heating value is too low and because of their fluctuating and high ash content, they change the quality of the cement to such an extent that it no longer complies with the quality requirements specified in the standards. Besides, low-grade fuels often have a high content of accompanying substances, which must not enter the cement or are allowed to enter it only in part. Finally, the burning in the rotary kiln and the calcining in the calciner result in a formation of nitrogen oxides, which must be removed from the exhaust gas.